1. Field of the Invention
The present invention relates to a process for producing dimethyltetralin. More particularly, it relates to a process for producing dimethyltetralin with a high yield over a long stabilized period by the cyclization of the corresponding 5-tolyl-penta-2-ene under specific reaction conditions by the use of a specific catalyst. Dimethylnaphthalene (hereinafter sometimes abbreviated as "DMN") which is obtained by the dehydrogenation of dimethyltetralin is a compound of utmost importance as a starting raw material for naphthalene dicarboxylic acid to be used in the production of plastics such as polyesters. For example, polyethylene-2,6-naphthalate which is produced from 2,6-naphthalene dicarboxylic acid and ethylene glycol has heat resistance and mechanical properties more favorable than polyethylene terephthalate and is used for producing films and fibers.
2. Description of the Related Arts
Isomerically high purity is required not only for naphthalene dicarboxylic acid as a starting "raw material for plastics but also for DMN as the starting raw material for the aforementioned acid. Specifically, DMN has 10 isomers according to the positions of methyl groups, 1,2-; 1,3-; 1,4-; 1,5-; 1,6-; 1,7-; 1,8-; 2,3-; 2,6-; and 2,7-DMN and, when used as the starting raw material for naphthalene dicarboxylic acid, it is required to be a specific highly pure DMN free from other position isomers.
As a process for producing DMN, there is available an isolation process from a high boiling fraction from petroleum refining or from the tar fraction of coal origin, naphthalene alkylation process, synthetic process using an alkylbenzene and an olefin and the like.
In the case of an isolation process from a high boiling fraction from petroleum refining or the tar fraction of coal origin, the DMN contained in each of the fractions is a mixture of various DMN isomers and therefore requires an isomerization step and troublesome isolation process for the purpose of obtaining a specific desired DMN from among the isomer mixture. With regard to isomerization, it is known that the above-mentioned 10 isomers are classified into 4 groups as mentioned hereinbelow and that the isomerization in the same group is relatively easy, whereas that among different groups is difficult. In addition, it is extremely difficult to isolate a specific desired DMN from various DMN isomers. Furthermore, a variety of components other than DMN that are contained in the above-mentioned fractions makes it extremely difficult to isolate and recover a specific desired DMN with high purity from the mixture of DMN and the others.
Group A 1,5-DMN; 1,6-DMN; and 2,6-DMN PA1 Group B 1,7-DMN; 1,8-DMN; and 2,7-DMN PA1 Group C 1,3-DMN; 2,3-DMN; and 1,4-DMN PA1 Group D 1,2-DMN PA1 T is the reaction temperature in degrees Celsius.
The alkylation process of naphthalene is put into practice usually using a solid acid as a catalyst such as zeolite and silica-alumina. The process, however, involves the problems in that there are produced monomethylnaphthalene, trimethylnaphthalene, etc. other than DMN, a high selectivity to DMN is not attained and the resultant DMN is a mixture of a number of isomers. Accordingly, the process makes it difficult to afford the specific desired DMN in high yield as is the case with the isolation process from a high boiling fraction from petroleum refining or the tar fraction from coal origin.
As a countermeasure against the aforementioned problems, there is available a process for producing a specific DMN from an alkylbenzene and an olefin through multistage steps, exemplified by Japanese Patent Application Laid-Open No. 96540/1990 in which 2,6-DMN is produced from m-xylene, propylene and carbon monoxide and U.S. Pat. No. 5,008,479 in which 2,6-DMN is produced from toluene, butene and carbon monoxide.
Similarly, Japanese Patent Application Laid-Open Nos. 134634/1974, 89353/1975 and 67261/1973 disclose a process for producing 5-(o-tolyl)-penta-2-ene from o-xylene and butadiene, a process for producing 1,5-dimethyltetralin by cyclizing 5-(o-tolyl)-penta-2-ene and a process for producing 1,5-DMN by dehydrogenating 1,5-dimethyltetralin, respectively. The combination of the above-disclosed processes enables the production of 1,5-DMN with isomerically high purity from o-xylene and butadiene.
Japanese Patent Application Laid-Open No. 503389/1989 discloses a process for producing highly pure 2,6-DMN by isomerizing 1,5-DMN into the mixture of 1,5-DMN, 1,6-DMN and 2,6-DMN, which mixture is crystallized into the objective 2,6-DMN. The aforesaid process is highly advantageous in that isomerization and crystallization are carried out among 3 DMN isomers belonging to the same group as compared with those among the isomers belonging to different groups.
2,6-DMN has attracted the highest attention recently among the DMN isomers as the starting raw material for 2,6-naphthalene dicarboxylic acid. Thus the emergence of a process for industrially producing 2,6-DMN is eagerly desired.
The aforesaid Japanese Patent Application Laid-Open No. 134634/1974 also discloses a process for producing 5-(p-tolyl)-penta-2-ene from p-xylene and butadiene. In this case, it is presumed that 1,7-DMN is obtained by the successive cyclization and dehydrogenation, 2,7-DMN is obtained in high purity by further isomerization and crystalization, and also the use of m-xylene enables the production of the mixture of 1,6-DMN and 1,8-DMN.
It can be said that the process for producing DMN by the use of xylene and butadiene as starting raw materials is industrially excellent, since it enables the production of a specific DMN with isomerically high purity as described hereinbefore.
The process for producing DMN from xylene and butadiene comprises the steps of synthesizing 5-tolyl-penta-2-ene by side-chain alkenylation, synthesizing dimethyltetralin by means of cyclization, synthesizing DMN by means of dehydrogenation, isomerizing DMN and crystalizing isolation. The synthesis of dimethyltetralin by cyclizing 5-tolyl-penta-2-ene is disclosed in Japanese Patent Application Laid-Open No. 93348/1974 in which is used a solid phosphoric acid as a catalyst and Japanese Patent Application Laid-Open No. 500052/1991 through PCT in which is employed as a catalyst a ultra-stabilized Y-type zeolite (USY zeolite) that is modified with platinum and copper, showing a yield of 95% or more in the working examples of both of the disclosures.
However, all the working examples turned out to be unfavorable with respect to the catalyst service life. The primary contributor to the shortened service life of the catalyst is the coking of the catalyst, which decreases the conversion efficiency of 5-tolyl-penta-2-ene. The decrease in the conversion efficiency of the starting raw material not only lowers the yield of the objective dimethyltetralin but also causes unreacted 5-tolyl-penta-2-ene to act as a catalyst poison in the following dehydrogenation step, thus further reducing the service life of the dehydrogenation catalyst.
With regard to the process for producing dimethyltetralin by the cyclization of the corresponding 5-tolyl-penta-2-ene, the conventional techniques as mentioned above suffer the disadvantage of shortened service life of the catalyst in spite of a high yield obtained.
Under such circumstances, intensive research and investigation were made by the present inventors in order to develop a process capable of producing dimethyltetralin with high conversion efficiency and high yield for a long-term stabilized period by the cyclization of the corresponding 5-tolyl-penta-2-ene. As a result, there have been found by the present inventors effective catalysts and a production process for the purpose. The present invention has been accomplished on the basis of the above-mentioned finding.